For oil field cementing operations to be successful, additives which reduce fluid loss are required to be added to the cement. Such additives will be used in well cementing operations where the bottom hole circulating temperatures (BHCT) may range from 80.degree.-170.degree. F., substantial salt concentrations may be present, and slurry retardation and viscosity are critical aspects as same affect pumpability and compressive strength.
The present invention relates to an aqueous cementing composition and method of using same in cementing oil and gas wells and the like. More particularly the present invention concerns incorporation of copolymers of N,N, dimethylacrylamide and 2-acrylamido, 2-methyl propane sulfonic acid in a hydraulic cement for the purpose of reducing fluid loss during cementing operations.
Certain polymer compositions have long been recognized by those skilled in the art of cementing wells in the petroleum industry as cementing additives useful in reducing fluid loss from a slurry of cement and water to the surrounding environment, i.e. the formation. These compositions are commonly referred to as "fluid loss additives."
An example of a fluid loss additive for use in an acidizing or fracturing composition is found in U.S. Pat. No. 4,107,057. In the '057 patent a copolymer of a sulfonic-acid modified acrylamide and a polyvinyl crosslinking agent is employed.
In the oil well cementing art, a variety of polymers have been disclosed as useful fluid loss additives for hydraulic oil well cements. For example, U.S. Pat. No. 4,015,991 discloses such a fluid loss additive for a hydraulic cement slurry consisting of hydrolyzed copolymers of acrylamide (AA) and 2-acrylamido, 2-methyl propane sulfonic acid (AMPS). However, these AA/AMPS copolymers are useful only in operations where the bottom hole circulating temperature (BHCT) ranges from 90.degree.-125.degree. F., whereas BHCT ranges encountered in such operations are often outside such a range. Still further, these copolymers have a salt tolerance of only up to about 10%.
The temperature limitations of the AA/AMPS copolymers, i.e. loss of usefulness above about 125.degree. F. BHCT, are believed to be the result of hydrolysis of the amide groups. The carboxylate groups formed by such hydrolysis convert the copolymers to materials which function to retard the setting of the cement and to reduce the compressive strength of the set cement. Further, in the lower portion of the above mentioned temperature range (between 90.degree.-100.degree. F.) the AA/AMPS copolymer is less effective as a fluid loss additive, requiring inclusion of larger amounts of such additive than at higher temperatures. The inclusion of sufficiently large amount of additive to create an acceptable fluid loss composition often creates viscosity and pumpability problems, since the addition of such copolymer directly affects the resultant slurry rheology. Copolymers of acrylamide and AMPS exhibit high viscosity and poor mixability, resulting in cement slurries having poor pumpability characteristics during cementing operations. Mixability is a subjective term used to describe how well the components in the cement composition wet and mix with each other, as well as the energy required to create a generally homogeneous slurry.
Hence, the industry desires a fluid loss additive that has as little effect on compressive strength, set time, viscosity, and thickening time as possible; is salt tolerable, i.e. does not exhibit substantial loss of effectiveness in the presence of salt; and is chemically stable during cementing operations. Further, such desired fluid loss additive should be compatible with as many other additives and environmental conditions as possible, should be soluble in cement slurries at normal ambient temperature encountered in oil well cementing operations, as well as to continue to provide fluid loss characteristics over broad temperature and cement pH ranges.
U.S. Pat. No. 4,404,111 discloses the use of copolymers of N,N, dimethylacrylamide and 2-acrylamido, 2-methyl propane- -sulfonic acid as viscosity control agents in aqueous compositions to facilitate petroleum recovery from subterranean bearing formations. The method of preparing said copolymers uses conventional free radical initiators such as ammonium persulfate and results in copolymers having average molecular weights of greater than about one million. Further, the amount of NNDMA monomer employed in preparing the AMPS/NNDMA copolymer is disclosed as between 70 to about 99.5 weight percent.